Geometric optimizations for reducing spontaneous emissions in photodiodes

Abstract

An optical structure that reduces the effects of spontaneous emissions from the active region of a laser. An optical structure includes optimizations to reduce the effects of spontaneous emissions. The optical structure includes a VCSEL with top and bottom DBR mirrors and an active region connected to the mirrors. The optical structure further includes a photodiode connected to the VCSEL. One or more optimizations may be included in the optical structure including optically absorbing materials, varying the geometry of the structure to change reflective angles, using optical apertures, changing the reflectivity of one or more mirrors, changing the photodiode to be more impervious to spontaneous emissions, and using ion implants to reduce photoluminescence efficiency.

1. An optical structure comprising:
 a VCSEL, the VCSEL comprising:
 a top DBR mirror; 
 an active region coupled to the top DBR mirror; and 
 a bottom DBR mirror coupled to the active region; 
 
 a photodiode coupled to the VCSEL; and 
 wherein the VCSEL includes a mesa structure, the mesa structure formed to include substantially regularly spaced irregularities on walls of the mesa structure, wherein the irregularities cause at least some spontaneous emissions to couple out of the VCSEL. 
 
     
     
       2. The optical structure of  claim 1 , wherein the mesa structure is formed by a wet etch process such that the mesa structure has frosted walls. 
     
     
       3. The optical structure of  claim 1 , wherein the irregularities are ridged irregularities and comprise lithographically formed ridges extending from a top end of the mesa structure to a bottom end of the mesa structure. 
     
     
       4. The optical structure of  claim 1 , further comprising an absorbing material disposed in the optical structure, the absorbing material comprising a composition configured to absorb optical emissions. 
     
     
       5. The optical structure of  claim 1 , wherein at least one of the top DBR mirror and the bottom DBR mirror is optimized to increase the critical angle caused by an interface between the DBR mirror and the active region. 
     
     
       6. The optical structure of  claim 1 , wherein at least one of the top DBR mirror and the bottom DBR mirror is optimized to reduce transmission of off-axis emissions at a specific angle. 
     
     
       7. The optical structure of  claim 1 , wherein the photodiode comprises an increased bandgap to reduce responsivity of the photodiode above a laser emission. 
     
     
       8. The optical structure of  claim 1 , further comprising a wide bandgap layer coupled to the photodiode on a side of the photodiode opposite the VCSEL. 
     
     
       9. The optical structure of  claim 1 , further comprising a DBR mirror on a side of the photodiode opposite the VCSEL. 
     
     
       10. A method of forming an optical structure, the method comprising:
 forming a VCSEL, wherein forming the VCSEL comprises:
 forming a top DBR mirror; 
 forming an active region coupled to the top DBR mirror; and 
 forming a bottom DBR mirror coupled to the active region; 
 
 forming a photodiode coupled to the VCSEL; and 
 wherein forming the VCSEL comprises forming a mesa structure, the mesa structure being formed so as to include substantially regularly spaced irregularities that cause at least some spontaneous emissions to couple out of the VCSEL; and 
 wherein the method yields an optical structure suitable for generating laser light and having substantially regularly spaced irregularities on the mesa structure. 
 
     
     
       11. The method of  claim 10 , wherein the irregularities comprise lithographically formed features. 
     
     
       12. The method of  claim 10 , further comprising forming an absorbing material disposed in the optical structure, the absorbing material comprising a composition configured to absorb optical emissions. 
     
     
       13. The method of  claim 10 , farther comprising optimizing at least one of the top DBR mirror and the bottom DBR mirror to increase the critical angle caused by an interface between the DBR mirror and the active region. 
     
     
       14. The method of  claim 10 , farther comprising optimizing at least one of the top DBR mirror and the bottom DBR mirror to reduce transmission of off axis emissions at a specific angle. 
     
     
       15. The method of  claim 10 , wherein the photodiode comprises an increased bandgap to reduce responsivity of the photodiode above a laser emission. 
     
     
       16. The method of  claim 10 , further comprising forming a wide bandgap layer coupled to the photodiode on a side of the photodiode opposite the VCSEL. 
     
     
       17. The method of  claim 10 , further comprising forming a DBR mirror on a side of the photodiode opposite the VCSEL. 
     
     
       18. The method of  claim 10 , further comprising forming an optical aperture between the VCSEL and photodiode to block a portion of off-axis spontaneous emissions, wherein the optical aperture is substantially surrounded by free space. 
     
     
       19. A method of forming an optical structure, the method comprising: forming a VCSEL, wherein forming the VCSEL comprises: forming a top DBR mirror; forming an active region coupled to the top DBR mirror; and forming a bottom DBR mirror coupled to the active region; forming a photodiode coupled to the VCSEL; wherein forming the VCSEL comprises forming a mesa structure, the mesa structure formed to include substantially regularly spaced irregularities on the walls of the mesa structure, wherein the irregular structures are formed lithographically with a photomask having irregularities corresponding to the irregular structures and the irregular structures cause at least some spontaneous emissions to couple out of the VCSEL, and wherein the method yields an optical structure suitable for generating laser light and having irregular structures on the walls of the mesa structure. 
     
     
       20. The optical structure of  claim 1 , further comprising an optical aperture between the VCSEL and photodiode to block a portion of off-axis spontaneous emissions, wherein the optical aperture is substantially surrounded by free space.